We are investigating the role of peptides as transmitters in the nervous system. The marine mollusk Aplysia provides a convenient experimental system for cellular physiological and biochemical studies of this problem. The abdominal ganglion of this animal contains a group of neuroendocrine cells, the bag cells, which synthesize and release peptides. We have found that an electrically triggered discharge of impulse activity in the bag cells produces several types of profound effects on neurons located near them in the abdominal ganglion. The effects, which include various types of excitation and inhibition, are specific to individually identifiable target neurons and they are unusually prolonged, lasting for minutes or hours. These interactions may represent a novel, nonsynaptic mode of communication between neurons. The putative hormonal transmitter for one of the interactions is egg laying hormone, a bag cell peptide that also induces egg laying. A second factor recently isolated from the bag cells is a candidate transmitter for mediating inhibitory effects. Our objectives are to continue to investigate the roles of these substances as transmitters, using the criteria that have been established for identification of other types of transmitters, and to investigate their cellular mechanisms of action on target neurons. Further analysis of these interactions may result in a clearer understanding of fundamental mechanisms of chemical signalling between neurons.